THE ROYAL ARTILLERY INSTITUTION. 
107 
cent, of moisture, the explosion does little or no damage. It is, therefore, 
a great advantage to work with as much moisture as possible. Some have 
thought that the more moisture in the powder when milled and pressed, the 
more moderate will be the results of proof, and that this is due to a sort of 
hard and crystalline texture which the powder thus assumes. The author does 
not believe in this theory, and certainly the results of proof do not bear it 
out. It is true no doubt that powder if too dry will not pack well together, 
even if pressed with a very high pressure; and this was found to be the case 
in the earlier experiments to obtain powder for heavy guns, when the Doremus 
pellets, which were pressed with as much as three tons to the inch, were 
found to be very violent even in field guns. A certain amount of moisture 
must therefore be in the powder when milled and pressed, in order to make 
it pack together; but if this amount is exceeded, the results are prejudicial, 
and, with pebble-powder, instead of mitigating the violence, rather increase 
it. No doubt, if the powder be not properly dried, the moisture has a 
moderating tendency, and probably this is the reason which has given rise to 
the crystalline theory. If, however, the powder be thoroughly dried, it has 
always been found that with moisture over a certain per-centage—about 4J 
or 5 per cent.—it is much more difficult to get a high density. No doubt 
this and the more violent action at proof are caused by the moisture, in forcing 
its way from the interior of the pebbles, forming porous channels, which not 
only swell the powder, but also leave passages for the flame to penetrate more 
readily into the interior. If too much moisture be in the powder when pressed, 
the water can be seen to be squeezed out, showing that the extra amount 
can produce no good effects, and must, if confined, prevent a high density 
being obtained. 
The great difficulty of getting the moisture from the interior of the pebbles, 
as has been stated, is another reason why too much moisture should not be 
used. In making these observations, however, the author begs to observe 
that they are entirely his own views, and are not necessarily accepted by 
everyone. 
A remark may, however, be made to which all that has been said seems 
naturally to tend—namely, that to produce a powder which shall be moderate 
in its action is both troublesome and expensive, unless an undue amount of 
moisture be admitted; but a powder can be produced safety, expeditiousty, 
and economically, which, though somewhat violent in its action, does not 
give an exceptionally high pressure in comparison with the velocity. If the 
guns will stand, no difficulty will be found in providing a powder capable of 
giving the very best results. 
The manufacture of pebble has been carried on in 1873 and 1874 much 
in the same manner. The variations of the seasons have to be studied, and, as 
a rule, it has been found that density is more easily obtained in summer than 
in winter, owing to the more effectual incorporation. The amount of milling 
has been reduced, as it appears unadvisable to prolong this operation—-which 
is the most dangerous of all—more than is absolutely necessary. 
The press-boxes at Waltham Abbey produce cakes of powder 30 ins. square, 
which are not only too large to pass through the pebble machine, but the size 
of the copper plates between the cakes allows the plates to buckle, and so pro¬ 
duces cakes of uneven thickness. The perfect action of the machine depends 
to a great extent on the uniform thickness of the cakes, otherwise the long 
